Point-to-multipoint fixed radio access systems consist of one base station, which is the central unit, and multiple User Terminals that exchange data with the base station. Usually a fixed length frame structure is used and, within such frame, uplink and downlink capacity is dynamically allocated.
The layer architecture of these systems consists of a Physical Layer, Medium Access Control (MAC) layer, a Convergence Layer (CL) and user layers. The main function of MAC layer is the radio resource management. Not all subscriber terminals, which share the upstream period on a demand basis, can transmit at the same time successfully as they can in a dedicated-medium situation. The base station MAC protocol determines who transmits and when, providing the appropriate transmission capacity.
Two basic duplexing methods can be supported by point-to-multipoint radio access systems: Time Division Duplex (TDD) and Frequency Division Duplex (FDD). The nature of the traffic being carried influences the choice of the duplex scheme.
The FDD provides two-way radio communication using paired radio frequency bands: one band for the transmission in the forward link, the other for the transmission in the reverse direction. For practical reasons, once the bandwidth dedicated to the two channels has been set, an immovable boundary in frequency/bandwidth is consequently set. These paired bands typically are of equal capacity. If the uplink/downlink bandwidth needs vary with time, sometimes the bandwidth will be wasted (low demand), and sometimes it will be inadequate (high demand). So this technique is ideal for symmetric communications in which the information flows in both directions are comparable in terms of capacity.
The TDD transmission technique requires a single carrier for full duplex communications. Transmit/receive separation occurs in the time domain as opposed to the frequency domain. Transmission direction alternates between downlink and uplink using a repetitive frame structure. Within such structure, the capacity of the carrier is divided between downlink and uplink transmission in direct proportion to the desired throughput. A 50% distribution between downlink and uplink time slots results in a symmetric, full duplex throughput. Moving the time boundary between downlink and uplink results in an asymmetric throughput and in an efficient accommodation of the channel requirements for bursty data traffic.
Summarising: FDD can adequately handle traffic that has relatively constant bandwidth requirements in both communication directions. On the other hand, TDD better manages time-varying uplink/downlink traffic because of the nature itself of the duplex scheme, which matches the traffic behaviour.
When the ratio between the allocated downlink frame portion and the uplink one varies in time, the TDD scheme is called dynamic or adaptive. The utilisation of adaptive TDD in fixed radio access systems involves an efficient use of the available spectrum when asymmetric and unpredictable traffic represents a considerable percentage of the traffic load of the system. Such dynamic TDD scheme has been depicted in FIG. 1. It is seen that the frame length comprises a fixed number of slots but that the split between up and downlink traffic varies.
Document “MAC Proposal for IEEE 802.16.1”, IEEE 802.16.1mc-00/10 of 2000-02-25 discloses a broadband communication standard proposal that makes use of TDD and various quality of service classes (QoS).
According to the above document, when a fixed radio access system adopts a dynamic TDD scheme, the split between uplink and downlink is a system parameter that is controlled at higher layers and that depends on the adopted Call Admission Control policies within the system. When the TDD split changes this is communicated to the MAC layer from higher layers via the control Service Access Point (SAP). Hence, the split movement is driven by services requesting bandwidth guarantees that the CAC takes care of.
For the above known point to multipoint fixed radio access TDD, the main function of the Medium Access Control (MAC) layer is the radio resource management. The Call Admission Control functionality resides at a higher layer (Network layer).
The Call Admission Control determines periodically the amount of bandwidth devoted to the uplink and downlink transmission. The MAC layer follows the information on the amount of bandwidth devoted to the uplink and downlink transmission coming from the Call Admission Control and allocates the uplink slots frame by frame to the different user terminals and the downlink slots frame by frame to the downlink traffic (traffic from the Base Station towards the user terminals). The Mac layer does its job without modifying the uplink/downlink amount of bandwidth, i.e. the position of the split once having been decided by the Call Admission Control is not moved by the Mac layer.
The known allocation procedure can be summarised by the following steps:    1. Periodically the Call Admission Control (CAC) analyses the guaranteed uplink and downlink traffic.    2. The Call Admission Control (CAC) evaluates the position of the split according to the guaranteed uplink and downlink traffic behaviour (if there have been no significant changes in the traffic the split position remains the same evaluated the period before).    3. The Call Admission Control (CAC) signals the information on the split position to the MAC layer.    4. The Mac layer allocates the resources frame by frame to the User terminals and to the Base station using its scheduling policy but always according to the amount of bandwidth devoted to the uplink and downlink transmission corresponding to the split evaluated at step 2.
The above TDD scheme does not take in consideration, for deciding the downlink/uplink split movement, the behaviour of the Best Effort traffic (or in general not guaranteed traffic). The term Best Effort refers to non real time services, usually-Internet services such as Web Browsing, E-mailing, FTP (File Transfer Protocol) and file sharing.
In the last few years, the demand in capacity for the latter type of services has increased substantially. Every category of user now wants Internet access as a basic service and the number of Internet connections continues to rise exponentially. If Internet access is the prime interest for a majority of customers then traffic will be very bursty and the overall downlink/uplink capacity ratio in the system may vary considerably.
In known dynamic TDD systems the bandwidth allocated to the best effort traffic is usually a fixed quantity that reflects the expected load of the Best Effort traffic evaluated only once during the dimensioning phase of the system. In this way the significant and unpredictable variations in time of the Best Effort traffic are not considered, and so it may happen that the downlink suffers of a lack of bandwidth, while the uplink is not using part of its allocated one, or vice versa.
Prior art document U.S. Pat. No. 5,602,836 shows a method for dynamically allocating bandwidth between up and downlink traffic in a TDMA system. The partition, also called split, between up and down-link slots in each frame is regulated, although the total number of slots are remains fixed for every frame. When few users use the system in relation to full capacity, the system operates like fixed partition TDD systems with equal up- and down-link slots in each frame. However, if the traffic in either direction exceeds half the available slots, the location of the split is adapted to the demand. If more than half the available slots are required in both directions, the split is set to half the available slots in both directions. The system adapts a circular interleaving method for separate queues for up and downlink traffic.
U.S. Pat. No. 5,768,254 shows a system aiming at reducing the runlength of dropped packets or the co-channel interference using adaptive TDD.